Electromechanical transducer system



Aug. 13, 1963 s. RICH Re. 25,433

ELECTROMECHANICAL TRANSDUCER SYSTEM Original Filed Aug. 27, 1956 2Sheets-Sheet 1 RELATIVELY HiGH DENSITY '7 MATERIAL 22 2 I flo RELATWELY/HG 3 LOW DENSITY MATERIAL INVENTOR.

STANLEY R. RICH Aug. 13, 1963 s, c Re. 25,433

ELECTROMECHANICAL TRANSDUCER SYSTEM Original Filed Aug. 27, 1956 2Sheets-Sheet 2 STRESS OR STRAIN [42 A 4 B 2 i FIG. 6 AMPLITUDE, VELOCITYOR ACCELERATION INVENTOR.

STANLEY R. RICH United States Patent Matter enclosed in heavy bracketsII] appears in the original patent but forms no part of this reissuespecification; matter printed Ill italics indicates the additions madeby reissue.

This invention relates to electromechanical transducers, and moreparticularly to systems employing such transduoers to interchange lagrequantities of energy with low losses.

Uses of elastic wave energy in industry are being made and innovated atan increasing pace. However in some areas, such as processes employingliquid baths which it is desired to irradiate with compressional wave(sonic or ultrasonic) energy, progress into large scale use of suchenergy awaits the availability of an electromechanical transducer atreasonable cost which will convert large amounts of electrical energyinto compressional wave energy, with small and insignificant heat loss.Known transducers generally lack the ability to handle large power,except perhaps as pulse peaks, without soon generating destructive heat.Etficiencies below 50% are common, and efficiencies as much as 65% areconsidered high. Further, the permissible stresses and strains that canbe endured by the electromechanical transducer materials are so low thatthese materials are soon destroyed by the amount of power which can beusefuly employed in such processes. Attempts to solve this problem byemploying more transducers often carries the cost of an installationbeyond economical acceptable limits.

It is an object of the present invention to provide an electromechanicaltransducer system which is capable of interchanging large amounts ofelectrical and elastic wave energy with high efficiency. It is anotherobject of the invention to provide such a system having a relatively lowQ so that it can be used with electric energy sources which do not havereliable frequency regulation, such as a motor generator driven by aninduction motor. It is a further object of the invention to provide sucha system which can operate continuously without generating destructiveheat in any of its parts. These and other desirable objects areattained, according to this invention, by a sandwich transducerstructure in which a thin electromechanical transducer is held betweentwo pieces of solid materials having relatively dissimilar densities,for example aluminum and brass, or aluminum and steel. According toanother important feature of the invention, this structure is heldtogether, by suitable clamping means, with a force sufficient to apply astatic compressive stress of a magnitude which may be chosen to begreater than and in oposition to any instantaneous negative stressproduced in the system during vibration at practically any energy level.These features of the invention are described in greater detail in thefollowing description of certain embodiments. This description refers tothe accompanying drawings, in which:

FIG. 1 is a vertical section through a transducer system built accordingto the invention;

FIG. 2 is a top view of FIG. 1;

FIG. 3 is a section along line 3-3 in FIG. 1;

FIG. 4 is a fragmentary vertical section showing a modification of FIG.1;

FIG. 5 illustrates in detail an electromechanical transducer elementsuitable for use in systems built according to the invention;

"ice

FIG. 6 is a graph to aid the description;

FIG. 7 is a vertical section, partly broken away through anotherembodiment of the invention; and

FIG. 8 illustrates another electromechanical transducer element suitablefor use in systems built according to the invention. 1

In FIG. 1 a block of steel or corresponding properties 10 and a block ofaluminum or other suitable light metal 11 have four electromechanicaltransducer elements, 12, 13, 14 and 15 as shown in FIG. 3, sandwichedbetween them. These may be made of any piezoelectric or electrostrictivematerial, for example quartz, or barium titanate. This sandwich is heldtogether by a bolt 17 passing through bores in the blocks 10 and 11. Thehead 18 of the bolt 17 rests on a shoulder provided by enlarging theouter portion 19 of the bore in the aluminum block 11. The outer portion21 of the bore in the steel block 10 is enlarged to receive anelectrically insulating sleeve 22 and the nut 23 of the bolt 17. Ashoulder 25 is provided at the junction of the outer portion 21 and theinner portion 26 of the bore in the steel block 10, and an electricallyinsulating washer 27 rests on this shoulder. A second electricallyinsulating sleeve 28 lines the inner bore portion 26. A convex shapedhard metal washer 3| rests on the washer 27, and the nut 23 is used tocompress this washer to apply a static compressive stress to the system.The magnitude of this stress, and its purpose are described below.

Referring to FIG. 4, the elements corresponding to elements shown inFIG. I bear the same reference numbers. The transducer elements (onlyelements 14 and 15 are shown) in this embodiment are affixed to theblocks 10 and 11 by a suitable cement 35. An epoxy cement is suitable.Each transducer element is fitted with electric conductors, one for eachface. The conductors 36 and 37 for element 15 only are shown, and theseare connected one to each of blocks 10 and 11. As indicated in FIG. 5,each transducer element is provided with an electrode 38 on each face;each electrode has a tab to which one of the wires 36 or 3-7 isattached. While in FIG. 1, the blocks 10 and 11 themselves makeelectrical contact with the transducer elements, which are usuallyfitted with electrodes like elctrode 38, these elements can if desiredbe fitted with conductors like conductors 36 and 37, and the conductorscan be bonded to the blocks as shown in FIG. 4, to insure directelectrical contact to the electrodes in all events. On the other hand,the conductors 36 and 37 may be omitted entirely from the embodimentshown in FIG. 4. I have built such systems, in which the staticcompressive stress was applied while the cement 35 was soft, and theyhave operated successfully.

The sandwich technique of transducer system construction was apparentlyinvented by Langevin-see FIG- URE 3 of British patent specification145,691, for example. In the Langevin system, two steel or stainlesssteel blocks of equal thickness are placed one on each side of a mosaicof quartz crystals. The system resonates as a half-wave vibrator withthe quartz (which is comparatively thin) located essentialy at a node.The overall Q of this device is extremely high, being basically theratio between the acoustic impedance of steel and the acoustic impedanceof the liquid in which it is used. This Q is of the order of 30. Thatis, the frequency span of operation is of the resonant frequency. At 20kc./sec., the Langevin transducer must be operated at plus or minus 330cycles per second in most liquids. This has too high a Q for industrialpurposes.

In the present invention, a low density (light) metal is used for oneside of the sandwich, and a high density (heavy) metal is used for theother side. Energy is radiated from the surface of the light metal. Oneeffect of this new structure is a reduction in Q, due to the fact othersuitable metal of that the acoustic impedanccs of light metals (aluminumor magnesium) are of the order of ,5; to that of steel, to values of Qof the order of 10. Conservation of momentum considerations stillfurther reduce the Q of the overall system by virtue of the fact thatthe surface of the light metal must move with a particle displacement ofmany times the particle displacement at the surface of the heavy metal.The ratio is the ratio of the densities in the two metals, if thevelocity of sound is the same in both. The reason for this is that thevelocity of sound determines the resonant frequency of the sand wichwhile the particle velocity is determined by the fact that the momentumof the two sides must be the same, to satisfy the requirements of thelaw of conservation of momentum. This dictates that the particlevelocities at the end surfaces of the two blocks and 11, for example,must be in inverse ratio to the densities of the two materials of theblocks, in order that the density of one metal times the particlevelocity at its surface may be equal to the density of the other metaltimes the particle velocity at its surface. In sandwich combinations ofsteel or brass for example as the heavy metal with aluminum or one ofits alloys as the light metal, a particle displacement or velocityamplification of about 3.33 is obtained, and when magnesium or one ofits alloys is substituted for aluminum or its alloys the particlevelocity or displacement amplification is about 5.6. The Q is thusfurther lowered because energy radiated from the surface of the lowerdensity metal is greater per unit energy stored in the resonantstructure.

Referring to FIG. 6, which illustrates schematically a half-wavelongitudinal vibrator 42, curve A indicates the amplitude, velocity oracceleration conditions at various particle positions in the vibrator ifit is constructed according to Langevin namely, a sandwich with bothfree ends made of solid materials having substantially the same density.These conditions are maximum and equal and opposite at the ends, passingthrough zero at the node, in the center. Curve B illustrates theparticle conditions provided by the present invention, that is, when thesystem is constructed as a sandwich in which the outer solid materialshave relatively different densities, or acoustic impedances. Assumingthe left-hand end to be that of the lower density metal, the maximumamplitude, velocity or acceleration of a particle at the lefthand end ofthe vibrator 42 is greater than at the righthand end, and greater forthe same input energy than in the prior art structures.

Momentum considerations in the present case cause the lighter metal,which is used as the radiating surface, to have greater particlevelocity than the heavy metal, as shown in FIG. 6. This means thatgreater energy is available for radiation because radiated energy isproportional to the square of particle velocity. Consequently thepresent invention, as distinguished from the prior art devices whichdistribute available energy equally to both halves of the sandwich,provides a far greater proportion of the total energy to the radiating(and lighter) part than to the non-radiating (heavier) side.

The radiated energy can be made to leave the transducer system (at thebottom surface 40 in FIG. 1, for example) in a plane-wave form. This isoften desirable because it can avoid undesirable focusing effects intreatment baths, and thereby permit accurately controlled irradiation ofsuch baths with arrays of transducer systems. As an example ofdimensions suitable for this purpose, one embodiment of a half-wavekc./sec. vibrator uses steel and aluminum alloy cubes 2" x 2" x 2", withbarium titanate A" thick between them. These dimensions of the cubes allapproximate to one quarter wave length of elastic waves in the material.If brass or other copper-bearing metal is used in place of the steel,its dimensions may be 2" x 2" x 1 /2", since brass and othercopper-bearing metals have a lower velocity of sound than aluminum and 1/2" depth is equivalent approximately to 2" in aluminum. The aluminumalloys used are alloys containing at least 70% aluminum.

While sandwich structures held together solely by cement will work andgive all the amplification effects described above, consideration ofcurve C in FIG. 6 will show that the stress or strain in a half-wavevibrator is maximum at the node. If the cement-ed surfaces are solocated in the structure, whether it be one-half wave long or longer,the strength of the cement bond limits the power that can be developedby the system.

When power is increased, eventually an instantaneous negative stresswill be reached which will be capable of weakening or rupturing thebond. The bolt 17 and nut 23 are preferably tightened on the washer 30to exert a static positive compressive stress which is greater inmagnitude than any instantaneous negative stress possible at desiredpower levels of operation. This makes it possible greatly to extend boththe power range and the useful life of sandwich transducer systems. Thebond ing material is prevented from going through the type of mechanicalhysteresis loop that occurs in non-engineering materials, which resultsin internal losses and eventual failure. As shown in FIG. 1, the bondingmaterial may be omitted entirely, when compressive stress is provided,if all meeting surfaces are accurately flat. I have built suchtransducer systems, and operated them suc cessfully.

The efficiency of transducer systems according to the invention is veryhigh, exceeding 90%, when barium titanate is used, for example.Ordinarily barium titanates, and certain ferrites, have efficienciesaround 65% when operated in a conventional thickness mode. The lossesare mainly mechanical (in barium titanate the electrical losses are lessthan 4%, as a capacitor). However, in the present invention, thetransducer material (e.g. barium titanate) is only a small fraction ofthe vibrating system. For example, in a 20 k-c./sec. system, it may beonly 4" out of 4", or 6.3% of the structure. Relatively lossless metalsor other materials then make up 94% of the total. The generated heat, orlosses, are quite small. Also, whatever heat is generated in thetransducer elements is easily given up to the metal masses because theelements are thin, and is easily carried away by the metal masses.

An alternative transducer system is shown in FIG. 7, in which a clampingmeans for applying static cornpressive stress to the system is locatedoutside the system. Blocks 50 and 51, corresponding respectively toblocks 10 and 11 in FIG. 1, hold between them transducer elements 54 and55 corresponding respectively to elements 14 and 15, in FIG. I. Flanges60 and 61 are provided on the left-hand side of each block, 50 and 51respectively, and similar flanges (not shown) are provided on theopposite side of each block. Each set of confronting flanges is providedwith bores, 62 and 63, and the bore 62 in flange 60 is fitted with anelectrically insulating sleeve 65. A bolt 67 is fitted in the bores andthrough the sleeve 65, and a curved metal washer 68, resting on anelectrically insulating washer 69, is compressed by a nut 71 to applythe static compressive stress. It will be appreciated that at least twosets of flanges and bolts are needed to apply this force, although todistribute it as evenly as possible more may be desired in a particularcase. Obviously, cement can be included according to FIG. 4, if desired.

FIG. 8 illustrates the structure of a magnetostrictive transducerelement which can be substituted in FIG. 1, FIG. 4 or FIG. 7, for theelements shown in FIG. 3. A block of magnetostrictive material isprovided with two bores 81 and 82 parallel to a pair of opposite sideedges. A wire 83 is coiled through these bores in a fashion to excitethe block into magnetostrictive vibration in the thickness dimension,when furnished with suitable electric current in a well known manner. Ahole 85 is provided in the center of the block, in the thicknessdirection, for passage of the bolt 17 if a structure like that of FIG. 1is used. The block 80 can be made of laminated sheets, or can be solid,as shown. It can be a ferrite. When magnetostrictive transducer elementsare used, the electrically insulating sleeves and washers, shown in FIG.1 and FIG. 7, may be omitted.

The embodiments illustrated and described herein are illustrations onlyof the invention, and other embodiments will occur to those skilled inthe art. No attempt has been made herein to go into all possibleembodiments, but rather only to "illustrate the principles of theinvention and the best manner now known to practice it.

What I claim is:

[1. An electromechanical transducer system comprising electromechanicaltransducer means sandwiched between two pieces of solid materials whichhave relatively dissimilar densities and constituting with said pieces amechanical vibrator, said system being dimensioned to vibrate as ahalf-Wave vibrator in the direction of a line through sa d pieces andsaid transducer means] 2. An electromechanical transducer systemcomprising electromechanical transducer means sandwiched between twopieces of solid materials which have relatively dissimilar densities andconstituting with said pieces a mechanical vibrator, said vibrator beingdimensioned to vibrate substantially as a longitudinally resonantvibrator in the direction of a line through said pieces and saidtransducer means, each of said pieces being dimensioned to constitutesubstantially one or more quarter-wave sections of said transducersystem.

3. A system according to claim 2 in which each of said pieces is lessthan one-half wave length long in any direction transverse to said lineat the vibrating frequency of the system.

4. An electromechanical transducer system comprising electromechanicaltransducer means sandwiched between two substantially cube shaped piecesof solid materials which have relatively dissimilar densities andconstituting with said pieces a mechanical vibrator, said pieces havinglength, height and width dimensions all approximating ito one-quarterwave length of elastic waves therein at the vibrating frequency of thesystem.

5. An electromechanical transducer system comprising electromechanicaltransducer means sandwiched between a piece of metal containing at least70% aluminum and a piece of copper-bearing metal.

6. An electromechanical transducer system comprising electromechanicaltransducer means sandwiched between a piece of metal containing at least70% aluminum and a piece of ferrous metal.

7. An electromechanical transducer system comprising electromechanicaltransducer means sandwiched between a substantially cube shaped piece ofmetal containing at least 70% aluminum and a substantially cube shapedpiece of ferrous metal.

8. An electromechanical transducer system comprising electromechanicaltransducer means sandwiched between a piece of metal containing asubstantial proportion of magnesium and a piece of copper-bearing metal.

9. An electromechanical transducer system comprising electromechanicaltransducer means sandwiched between a piece of metal containing asubstantial proportion of magnesium and a piece of ferrous metal.

10. An electromechanical transducer system comprising electromechanicaltransducer means sandwiched between a substantially cube shaped piece ofmetal containing a substantial proportion of magnesium, and asubstantially cube shaped piece of ferrous metal.

11. An electromechanical transducer system comprising piezoelectricmaterial sandwiched between a piece of metal containing at leastaluminum and a piece of ferrous metal.

12. An electromechanical transducer system comprising electrostrictivematerial sandwiched between a piece of metal containing at least 70%aluminum and a piece of ferrous metal.

1.3. An electromechanical transducer system comprising magnetostrictivematerial sandwiched between a piece of metal containing at least 70%aluminum and a piece of ferrous metal.

14. An electromechanical transducer system comprising piezoelectricmaterial sandwiched between a piece of metal containing a substantialproportion of magnesium and a piece of comparatively higher densitymetal and constituting with said pieces a mechanical vibrator.

15. An electromechanical transducer system comprising electrostrictivematerial sandwiched between a piece of metal containing a substantialproportion of magnesium and a piece of comparatively higher densitymetal and constituting with said pieces a mechanical vibrator.

16. An electromechanical transducer system comprising magnetostrictivematerial sandwiched between a piece of metal containing a substantialproportion of magnesium and a piece of comparatively higher densitymetal and constituting with said pieces a mechanical vibrator.

17. An electromechanical transducer system comprising electromechanicaltransducer means sandwiched between two pieces of solid material whichhave relatively dissimilar densities and constituting with said picccs amechanical vibrator, said system being dimensioned to vibrate as ahalf-wave vibrator in the direction of a line through said pieces andsaid transducer means, the piece having the greater density being ofgreater mass than the piece of lesser density, and said pieces eachhaving along said line the same ratio of dimension to wave lengthvibration therein.

18. An electromechanical transducer system comprising electromechanicaltransducer means sandwiched between two pieces of solid material whichhave relatively dissimilar densities and constituting with said pieces amechanical vibrator, said system being dimensioned to vibrate as ahalf-wave vibrator in the direction of a line through said pieces andsaid transducer means, the piece having the greater density also beingof such greater mass than the piece of lesser density that the ratio ofthe mass of the piece of greater density with respect to the mass of thepiece of lesser density is not less than 3, said pieces beingdimensioned along said line so as each to have at vibration the somenumber of approximately quarterwave lengths along said line.

References Cited in the file of this patent FOREIGN PATENTS 145,691Great Britain July 28,

